It is known that plastics materials are used more and more as insulators in manufacturing medium and high-tension electric cables. One of the commonest plastics materials used is reticulated or non-reticulated polyethylene. In the case of underground cables and especially in the case of submarine cables, the dielectric properties of polyethylene are in danger of being deteriorated by the ingress of dampness which promotes in particular the formation of arborescences and can lead to breakdown.
It is therefore necessary to provide a sealing sheath.
The cables may be sealed by means of an extruded metal sheath, e.g. made of lead as has been done since the earliest electric cables, or made of aluminium, or even by means of a metal (copper, aluminium or steel) tape laid lengthwise and folded over the cable. The edges of the tape may be welded, or clipped, or merely made to overlap with or without adhesive fixing. Such a tape is generally covered, at least on one surface, with a plastics material which adheres to the insulator or to the outer protective sheath or to both simultaneously, and may adhere to both edges if they overlap. The sheath produced by means of a metal tape leads to a lighter and more economical cable structure. When it is required to impart a degree of flexibility to the cable, a transversally corrugated tape is used.
Unfortunately, the various types of sheath listed hereinabove have no radial elasticity. Now, polyethylene has a high coefficient of expansion and if full advantage is to be taken of the excellent heat resistance of polyethylene and especially of reticulated polyethylene, the sealing sheath must be able to withstand great expansion between minimum ambient temperature and maximum operation temperature without detriment to transversal and longitudinal sealing and without any unsticking which would promote ionization. In other words, the sealing sheath must follow the radial deformation of the insulator.
Preferred embodiments of the present invention provide a solution to this problem and allow the metal sheath to follow the radial deformation of the polyethylene insulator when the cable is used as a high-power conductor subject to thermal losses.
The present invention provides a sealed power cable which comprises, from its centre to its periphery, a stranded conductor which is covered with a first or inner screen of semiconductor material which is coated with an insulator which is surrounded by a second or outer screen of semiconductor material which is enveloped in a metal strip and with an outer sheath of plastics material, wherein the said metal strip has longitudinal corrugations in order to allow thermal expansion without affecting the sealing of said metal strip and that sealing means are disposed between the "troughs" of the corrugations of the metal strip and the second or outer semiconductor screen.
Embodiments of the invention are described with reference to the accopanying drawings.